A circular cylindrical shaft which is subjected to a torque is influenced by a pure shear stress. This stress state can be expressed, in terms of its principal stresses, as a compressive stress and a tensile stress, directed perpendicularly thereto, of the same magnitude. The principal stress directions are inclined at .+-.45.degree. to the longitudinal axis of the shaft.
The state of the art as regards the constructive design of torque transducers is disclosed in a number of patent specifications and technical articles. Common to most of these solutions is that two zones are created in the magnetic material, with some type of anisotropy, which causes the magnetic flux density to be deflected at an angle away from its natural direction in parallel with the axis of the transducer shaft. In one zone the principal direction of the anisotropy coincides with the principal direction which provides tensile stress. In the other zone, the principal direction coincides with the direction which provides compressive stress.
Because of the magnetoelastic effect (in the case of positive magnetostriction), the zone reluctance will therefore decrease or increase if the magnetic flux density has been deflected towards the tensile direction or the compressive direction.
By finally measuring the difference in reluctance between these zones, a measure of the torque is obtained which has little sensitivity to axial forces or bending stress.
The reluctance difference is usually measured by creating a time-dependent H-field directed along the shaft and with equal amplitude in both zones, using a primary coil concentric with the shaft. With the aid of two identical secondary coils, centred around each zone, the difference in B-fields between the zones can be measured. This is achieved in the simplest way by connecting the secondary coils in opposition in such a way that the induced voltages in the respective coil are subtracted from each other. By phase-sensitive rectification of the secondary signal obtained in this way, it is possible, in addition, to distinguish between torsional moments of different directions.
This reluctance-measuring part in many cases constitutes a kind of magnetic yoke to the shaft and will, therefore, in the following be referred to as the "yoke".
To create a high sensitivity to torsional moments, it is required that the anisotropy is sufficient, such that the difference between the zones becomes as great as possible. A measure of the anisotropy is the angle at which the magnetic field is deflected from the natural direction parallel to the axis of the transducer shaft because of the influence of the anisotropy. If this angle is 45.degree. in the zones, the anisotropy is optimal as the B-field is then directed along the principal stress directions of the transducer shaft loaded with torsion.
Of the utmost importance is maintaining a complete rotational symmetry, both with respect to the distribution of the mechanical stress and the magnetic field, in order to prevent a signal variation which is only due to the transducer being rotated in relation to the reluctance-measuring part.
What distinguishes different torque transducers according to the above general description is primarily the method of realizing the anisotropy.
SU 667836 describes a method in which the anisotropy is created purely geometrically in each zone by cutting grooves in the surface of the shaft according to a specific pattern. This pattern consists of a number of mutually parallel lines directed at an angle of 45.degree. to the axis of the transducer shaft.
However, this solution entails an insufficient anisotropy and hence also low sensitivity, since the magnetic field can "creep under" the grooves in a relatively simple manner, unless these grooves are made deep. If the grooves are made deep, however, the stress level in the surface of the shaft, and hence also the sensitivity, will be lowered.
In addition, the grooves in the surface lead to greatly increased effective stresses in the bottom of the grooves and, therefore, the shaft can only be loaded to a moderate extent before plastic yielding of the shaft material sets in, which in turn leads to hysteresis in the output signal of the transducer.
U.S. Pat. No. 4,823,620 describes the same embodiment as above with respect to the geometrical anisotropy, however with the addition that the surface of the shaft is hardened or carburized for the purpose of reducing the hysteresis in the transducer.
In this U.S. patent specification it is also pointed out that it is not necessary to provide grooves in the surface of the shaft. It is also possible to create elevations in the shaft surface, or lands according to the above-mentioned pattern.
Further, the material in the above-mentioned lands or strips shall be non-magnetic and preferably a material with high electrical conductivity represented by copper.
One requirement on the strips according to U.S. Pat. No. 4,823,620, however, is that the width and thickness thereof should exceed the magnetic penetration depth, that is the skin depth, of the underlying material.
However, this requirement entails a considerable limitation of what is realizable in practice. A typical transducer material with a relative permeability of 120, a resistivity of 60.10.sup.-8 .OMEGA.meter, and a supply frequency of 2000 Hz will have a skin depth of 0.8 mm according to the accepted definition described below. In electrolytic plating of, for example, printed circuit-boards, the maximum rate of deposition is about 0.04 mm per hour. Plating a layer thicker than the skin depth would consequently in this case take far too long; instead, it is necessary, for example, to paste copper strips, which is hardly practical.
In addition, strips thicker than the skin depth have proved to entail considerable disadvantages as regards the temperature properties of the transducer.
One such property is the signal of the transducer in an unloaded state, that is, its zero signal. However, it is often possible to reset this signal at the temperature in question, in which case the temperature drift of the zero signal becomes less important.
Another transducer property, which is more difficult to eliminate, is the temperature dependence of the sensitivity of the transducer to torsional moments, the so-called sensitivity drift.
One known method of compensating for the above-mentioned sensitivity drift is to voltage-divide the signal with the aid of a network of temperature-dependent resistances or thermistors. An equivalent method is to simply load the secondary winding with a resistor, whereby essentially a voltage division is obtained between the copper resistance in the secondary winding and the external resistor.
Common to the above known methods of compensating the sensitivity drift is that they are costly and that the voltage division of the signal deteriorates both its output impedance and the signal level, hence increasing the sensitivity of the signal to disturbance.
Another and more serious drawback is that the sensitivity drift due to the temperature of the transducer shaft is compensated with a temperature drift in electric components in the reluctance-measuring part or at some other location, where the temperature may be completely different from the temperature of the shaft.
It would, therefore, be desirable to design a measuring zone which is self-compensating as regards the temperature drift of the sensitivity.
Since magnetoelastic transducers utilize a material property which is difficult to control accurately when preparing the material, the sensitivity of such transducers to mechanical stresses will depend on from which material batch the transducer is manufactured. This means that the sensitivity must be trimmed for each transducer, normally with the aid of voltage division of the secondary signal, which, as in the case with compensation of the sensitivity drift according to the above, leads to increased costs and increased sensitivity to disturbance.
Since by doing so a property of the shaft is compensated with a property of the reluctance-measuring yoke, this method also leads to the transducer shaft and the yoke not being replaceable between different transducers. This means that transducers and yokes must be kept in stock, handled, installed and possibly be replaced as one unit, which leads to higher costs and to a risk of the wrong shaft after all being paired together with the wrong yoke, resulting in incorrect sensitivity to torsional moments of the transducer.
It would, therefore, be desirable to design a measuring zone whose sensitivity to torsional moments can be trimmed in a simple manner depending on the material batch from which the magnetoelastic surface layer of the measuring zone is manufactured.
The above-mentioned economic and practical aspects may have a decisive importance when a torque transducer is to be used in applications requiring mass production and far-reaching rationalization, for example when measuring torque in cars.